Apparatus for maintaining color balance despite variations of the supply voltage

ABSTRACT

IN A THREE-GUN COLOR DISPLAY TUBE AT LEAST ONE OF THE CATHODES IS CONNECTED TO THE LUMINANCE AMPLIFIER THROUGH A NONLINEAR NEGATIVE FEEDBACK RESISTIVE CIRCUIT, WHICH IS AT LEAST PARTIALLY SHUNTED BY A UNILATERALLY CONDUCTING ELEMENT.

iJniTn inventors Filed Patented Assignee Priority Leonnrdus Aclrinnus.Tolinnnes Verlhoeven; lHlendr-ikns Dollelknmp. Emmnsingel, Eindlhoven,Netherlands 5cm. 22, B969 J1 une 28, W7 11 US. Philips Corporation NewTor-Tr, NY.

Sept. 21, 11968 Netherlnncls APPARATUS FOR MAllNTAlINllNG COLON BALANCElDESPTTE VAMATTONS UP THE SIUPPILT VOLTAGE 2 Clnims, 1 Drawing Pig.

llni.Cl...... Pield oi Search References Cited UNITED STATES PATENTS3,283,065 11/1966 Boekhorst Primary Examiner- Richard Murray Assislan!Examiner-John C. Martin Attorney-Frank R. Trifari ABSTRACT: In athree-gun color display tube at least one of the cathodes is connectedto the luminance amplifier through a nonlinear negative feedbackresistive circuit, which is least partially shunted by a unilaterallyconducii 11;; 0154mm.

PATENTEUJUNZEHSTE 3,588,328

INVENTORS LEONARDUSIMJ. VERHOEVEN BY HENDR KUS DOLLEKAMP AGENTAPPAIAA'II'IUS 1801M MAINTAINING COLON BALANCIE IDIESPITIE VARIATIONS UPTHE SUPPLY VOLTAGE In a three-gun color display tube at least one ofthecathodes is connected to the luminance amplifier through a nonlinearnegative feedback resistive circuit, which is at least partially shuntedby a unilaterally conducting element.

The invention relates to a display device for a color television systemincluding a display tube at least one of the cathodes of which isconnected with respect to direct current through a nonlinear negativefeedback resistive circuit to the output electrode of a luminance signalamplifier, said output electrode furthermore being connected through aresistor to a supply source.

A display device of the type described above is known from Dutch Pat.application 292,267. A very satisfactory color balance in the picturedisplayed is maintained in a simple manner with the aid of such a deviceupon variations in the supply voltage. However, in such a display deviceunwanted aftereffects in the form of so-called color smears may becomevisible upon some color transitions in a picture displayed.

It is an object of the invention to limit these aftereffects as much aspossible.

To this end a display device of the kind described in the preambleaccording to the invention is characterized in that the nonlinearnegative feedback resistive circuit is at least partly shunted by anelement unilaterally conducting in the direction from the outputelectrode to the cathode.

The applicant has found that the aftereffects are the result of theparasitic capacitance between the cathode and the associated controlelectrode of the gun (or the guns) in the cathode line which include anonlinear negative feedback resistor, In fact, the nonlinear resistorhas a large value when the cathode of the relevant gun is greatlypositive relative to the control electrode and has a smaller value whenthe cathode is less positive. Upon a variation of the voltage differencebetween the relevant cathode and the corresponding control electrode, atwhich the cathode becomes more positive relative to the controlelectrode, the time constant of the series arrangement of the nonlinearnegative feedback resistor and the parasitic capacitance between thecathode and the control electrode increases, so that this capacitance isrecharged more slowly. A positive going step of the voltage at theoutput electrode of the luminance signal amplifier relative to that atthe associated control electrode cannot be followed sufficiently quicklyby the relevant cathode, unless the step according to the invention istaken.

An unilaterally conducting element provided across the voltage dependentcathode-resistor according to the invention recharges the said parasiticcapacitance, when the voltage at the cathode is lower than that at theoutput electrode of the Inminance signal amplifier. This results in aquick following of the cathode voltage during the above-mentionedvoltage steps.

In order that the invention may be readily carried into effect, anembodiment thereof will now be described in detail by way of examplewith reference to the accompanying diagrammatic drawing.

In the drawing, which has only one FIGURE, details which are notimportant for a good understanding of the invention have been omitted.

The FIGURE illustrates by way of a nondetailed diagram a connectionbetween a luminance signal output value and the cathodes of a colortelevision picture display tube according to the invention.

An anode of a luminance signal amplifier output value l to the controlgrid of which a luminance signal Y is applied through an input 2, isconnected through a resistor 3 to a positive supply voltage source. Theanode of the valve 1 is furthermore directly connected to a cathode 4 ofa picture display tube 5. A second cathode 6 of the picture display tubeis connected to the anode of the value l through a parallel arrangementof a capacitor 7 and a resistor 13 shunted by a diode 9 and a resistor11 according to the invention. A third cathode 141 of the picturedisplay tube 5 is connected to the anode of the valve 1 through aparallel arrangement of a capacitor 15 and a nonlinear resistor 21shunted, according to the invention, by a diode l7 and a resistor 19.

The picture display tube 5 has three control electrodes 22, 23 and 25which are arranged opposite the cathodes A, 6 and M, respectively. Thecontrol electrodes are connected to inputs 27, 29 and 31 to which colordifference signals R-Y, G-Y and B-Y are applied from sources havingimpedances which are low at least with respect to high frequencies. Thecontrol electrodes 22, 23 and 25 are furthermore connected throughresistors 33, 35 and 37 to an adjustable tapping on a potentiometer 39connected to a supply source. The luminance of the picture to bedisplayed can be adjusted with the aid of the potentiometer 39.

As a result of the nonlinear negative feedback resistors 13, 21 in thecathode lines of the cathodes 6 and 14, it is achieved that thesensitivity of the three electrode systems of the display tube 3 issubstantially equal. In case of supply voltage variations the correctcolor balance is therefore maintained on the display tube 5.

The operation of the circuit arrangement will now be described in so faras this is important for the understanding of the invention.

The first cathode 4 of the picture display tube 5 and the controlelectrode 22 are supplied by a signal source having a low impedance. Thesecond and third cathodes 6 and 14 are each supplied through animpedance by the output valve 1. As a result of parasitic capacitance(shown in the FIGURE by a broken line and indicated by the referencenumerals All, 43 and 415) between the corresponding cathodes and controlelectrodes, these mentioned impedances exert influence on the display ofsteplike signals which are supplied to the cathodes 6 and M and thecontrol electrodes 23 and 25. As an example will now be checked thebehavior of the cathodewehnelt combination 6, 23 in cooperation with theimpedance formed by the parallel arrangement 7,. 9, 11 and 13, When thecathode 6 is slightly more positive than the control electrode 23, muchcurrent flows through the nonlinear resistor 13 and its resistance issmall. The value of the resistor 11 is now large relative to that of thenonlinear resistor 13. In a stationary condition the capacitor 7 and thediode 9 do not convey current. In fact, the diode 9 is connected in sucha manner that is is blocked when a current flows from the cathode 4 tothe anode of the valve ll, which will always be the case in thestationary condition. If the cathode 6 becomes more positive relative tothe control electrode 23, little current flows through the nonlinearresistor 13 and its resistance is large. If the influence of theresistor 11 is left out of consideration, the behavior of the circuitarrangement in case ofa sudden transition from the one above-describedstationary condition to the other is as follows. For a transition atwhich first little and then much current flows through the cathode 6, anegative voltage step at the anode of the valve 1 must occur relative tothe signal source connected to the input 29. Part of this voltage stepin a negative direction is immediately transferred by the capacitor 7 tothe parasitic capacitor 43, whereafter the capacitor 43 is quicklyfurther discharged through the then low resistance of the nonlinearresistor 13. The diode 9 thus remains blocked all the time.

If a voltage step in a positive direction occurs between the anode ofthe valve 1 and the signal source connection 29 for the purpose ofchanging a large cathode current into a small one, the value of thenonlinear resistor 13 will increase so that it will not be able toquickly charge the parasitic capacitor 43. At the beginning of thevoltage step the voltage across the cathode 4i and the parasiticcapacitor 43 will remain lower than that across the anode of thevalve 1. The diode 9 then conducts and the parasitic capacitor 43 isquickly charged through the diode 9. Without this diode 9 a positivegoing voltage step could not be followed sufficiently quickly by theparasitic capacitor 43 and hence by the cathode 6. This could becomevisible in the displayed picture as an aftereffect, the so-called colorsmears. In case of small positive going voltage steps the diode 9 is notvery active due to the curvature of its characteristic. The resistor 11connected in parallel with the diode 9 provides a somewhat quickercharge of the parasitic capacitor 43 in case of these small voltagesteps and consequently a decrease of the aftereffect.

The influence of the diode l7 and the resistor 19 for voltage stepsbetween the cathode 14 and the control electrode 25 can be explained inan analogous manner.

In the foregoing a parallel arrangement is described of a diode 9, l7and a resistor l1, 19 to improve the transfer of positive going voltagesteps from the anode of the valve 1 to the cathode 6, 14 through thenonlinear resistor 13, 17. lt will be evident that any combination ofelements which has a nonlinear characteristic equal to that of theparallel arrangement of said elements will be usable to prevent the saidaftereffects. It will furthermore be evident that the diodes 9 and 17may be connected across part of the nonlinear resistors 13 and 21 ifthese are built up from a number of components.

We claim:

1. A display device for a color television system including a displaytube at least one of the cathodes of which is connected with respect todirect current through a nonlinear negative feedback resistive circuitto the output electrode of a luminance signal amplifier, said outputelectrode being furthermore connected through a resistor to a supplysource, characterized in that the nonlinear negative feedback resistivecircuit is at least partly shunted by an element unilaterally conductingin the direction from the output electrode to the cathode.

2. A display device as claimed in claim 1, wherein the unilaterallyconducting element is a diode, characterized in that the diode isshunted by a resistor.

